This invention relates to processes for fluid separation utilizing membranes formed from crystalline titanium silicate molecular sieves.
Since the discovery by Milton and coworkers (U.S. Pat. Nos. 2,882,243 and 2,882,244) in the late 1950""s that aluminosilicate systems could be induced to form uniformly porous, internally charged crystals, analogous to molecular sieve zeolites found in nature, the properties of synthetic aluminosilicate zeolite molecular sieves have formed the basis of numerous commercially important catalytic, adsorptive and ion-exchange applications. This high degree of utility is the result of a unique combination of high surface area and uniform porosity dictated by the xe2x80x9cframeworkxe2x80x9d structure of the zeolite crystals coupled with the electrostatically charged sites induced by tetrahedrally coordinated Al+3. Thus, a large number of xe2x80x9cactivexe2x80x9d charged sites are readily accessible to molecules of the proper size and geometry for adsorptive or catalytic interactions. Further, since charge compensating cations are electrostatically and not covalently bound to the aluminosilicate framework, they are generally base exchangeable for other cations with different inherent properties. This offers wide latitude for modification of active sites whereby specific adsorbents and catalysts can be tailor-made for a given utility.
In the publication xe2x80x9cZeolite Molecular Sievesxe2x80x9d, Chapter 2, 1974, D. W. Breck hypothesized that perhaps 1,000 aluminosilicate zeolite framework structures are theoretically possible, but to date only approximately 150 have been identified. While compositional nuances have been described in publications such as U.S. Pat. Nos. 4,524,055; 4,603,040; and 4,606,899, totally new aluminosilicate framework structures are being discovered at a negligible rate.
With slow progress in the discovery of new aluminosilicate based molecular sieves, researchers have taken various approaches to replace aluminum or silicon in zeolite synthesis in the hope of generating either new zeolite-like framework structures or inducing the formation of qualitatively different active sites than are available in analogous aluminosilicate based materials.
It has been believed for a generation that phosphorus could be incorporated, to varying degrees, in a zeolite type aluminosilicate framework. In the more recent past (JAC 104, pp. 1146 (1982); proceedings of the 7th International Zeolite Conference, pp. 103-112, 1986) E. M. Flanigan and coworkers have demonstrated the preparation of pure aluminophosphate based molecular sieves of a wide variety of structures. However, the site inducing Al+3 is essentially neutralized by the P+5, imparting a +1 charge to the framework. Thus, while a new class of xe2x80x9cmolecular sievesxe2x80x9d was created, they are not zeolites in the fundamental sense since they lack xe2x80x9cactivexe2x80x9d charged sites.
Realizing this inherent utility limiting deficiency, for the past few years the research community has emphasized the synthesis of mixed aluminosilicate-metal oxide and mixed aluminophosphate-metal oxide framework systems. While this approach to overcoming the slow progress in aluminosilicate zeolite synthesis has generated approximately 200 new compositions, all of them suffer either from the site removing effect of incorporated p+5 or the site diluting effect of incorporating effectively neutral tetrahedral +4 metal into an aluminosilicate framework. As a result, extensive research has failed to demonstrate significant utility for any of these materials.
A series of zeolite-like xe2x80x9cframeworkxe2x80x9d silicates have been synthesized, some of which have larger uniform pores than are observed for aluminosilicate zeolites. (W. M. Meier, Proceedings of the 7th International Zeolite Conference, pp. 13-22 (1986)). While this particular synthesis approach produces materials which, by definition, totally lack active, charged sites, back implantation after synthesis would not appear out of the question although little work appears in the open literature on this topic.
Another and most straightforward means of potentially generating new structures or qualitatively different sites than those induced by aluminum would be the direct substitution of some charge inducing species for aluminum in a zeolite-like structure. To date the most notably successful example of this approach appears to be boron in the case of ZSM-5 analogs, although iron has also been claimed in similar materials. (EPA 68,796 (1983), Taramasso, et. al.; Proceedings of the 5th International Zeolite Conference; pp. 40-48 (1980)); J. W. Ball, et. al.; Proceedings of the 7th International Zeolite Conference; pp. 137-144 (1986); U.S. Pat. No. 4,280,305 to Kouenhowen, et. al. Unfortunately, the low levels of incorporation of the species substituting for aluminum usually leaves doubt if the species are occluded or framework incorporated.
In 1967, Young in U.S. Pat. No. 3,329,481 reported that the synthesis of charge bearing (exchangeable) titaniumsilicates under conditions similar to aluminosilicate zeolite formation was possible if the titanium was present as a xe2x80x9ccritical reagentxe2x80x9d +III peroxo species. While these materials were called xe2x80x9ctitanium zeolitesxe2x80x9d no evidence was presented beyond some questionable X-ray diffraction (XRD) patterns and his claim has generally been dismissed by the zeolite research community. (D. W. Breck, Zeolite Molecular Sieves, p. 322 (1974); R. M. Barrer, Hydrothermal Chemistry of Zeolites, p. 293 (1982); G. Perego, et. al., Proceedings of 7th International Zeolite conference, p. 129 (1986)). For all but one end member of this series of materials (denoted TS materials), the presented XRD patterns indicate phases too dense to be molecular sieves. In this case of the one questionable end member (denoted TS-26), the XRD pattern might possibly be interpreted as a small pored zeolite, although without additional supporting evidence, it appears extremely questionable.
A naturally occurring alkaline titanosilicate identified as xe2x80x9cZoritexe2x80x9d was discovered in trace quantities on the Siberian Tundra in 1972 (A. N. Mer""kov, et. al.; Zapiski Vses Mineralog. Obshch., pp. 54-62 (1973)). The published XRD pattern was challenged and a proposed structure reported in a later article entitled xe2x80x9cThe OD Structure of Zoritexe2x80x9d, Sandomirskii, et. al., Sov. Phys. Crystallogr. 24(6), November-December 1979, pp. 686-693.
No further reports on xe2x80x9ctitanium zeolitesxe2x80x9d appeared in the open literature until 1983 when trace levels of tetrahedral Ti(IV) were reported in a ZSM-5 analog. (M. Taramasso, et. al.; U.S. Pat. No. 4,410,501 (1983); G. Perego, et. al.; Proceedings of the 7th International Zeolite Conference; p. 129 (1986)). A similar claim appeared from researchers in mid-1985 (EPA 132,550 (1985)). The research community reported mixed aluminosilicate-titanium (IV) (EPA 179,876 (1985); EPA 181,884 (1985) structures which, along with TAPO (EPA 121,232 (1985) systems, appear to have no possibility of active titanium sites. As such, their utility has been limited to catalyzing oxidation.
In U.S. Pat. No. 4,938,939, issued Jul. 3, 1990, Kuznicki disclosed a new family of synthetic, stable crystalline titanium silicate molecular sieve zeolites, which have a pore size of approximately 3-4 Angstrom units and a titania/silica mole ratio in the range of from 1.0 to 10. The entire content of U.S. Pat. No. 4,938,939 is herein incorporated by reference. Members of the family of molecular sieve zeolites designated ETS-4 in the rare earth-exchanged form have a high degree of thermal stability of at least 450xc2x0 C. or higher depending on cationic form, thus rendering them effective for use in high temperature catalytic processes. ETS zeolites are highly adsorptive toward molecules up to approximately 3-5 Angstroms in critical diameter, e.g. water, ammonia, hydrogen sulfide, SO2, and n-hexane and are essentially non-adsorptive toward molecules, which are larger than 5 Angstroms in critical diameter.
A large pore crystalline titanium silicate molecular sieve composition having a pore size of about 8 Angstrom units has also been developed by the present assignee and is disclosed in U.S. Pat. No. 4,853,202, which patent is herein incorporated by reference. This crystalline titanium silicate molecular sieve has been designated ETS-10.
The new family of microporous titanium silicates developed by the present assignee, and generically denoted as ETS, are constructed from fundamentally different building units than classical aluminosilicate zeolites. Instead of interlocked tetrahedral metal oxide units as in classical zeolites, the ETS materials are composed of interlocked octahedral chains and classical tetrahedral rings. In general, the chains consist of six oxygen-coordinated titanium octahedra and wherein the chains are connected three dimensionally via tetrahedral silicon oxide units or bridging titanosilicate units. The inherently different crystalline titanium silicate structures of these ETS materials have been shown to produce unusual and unexpected results when compared with the performance of aluminosilicate zeolite molecular sieves. For example, the counter-balancing cations of the crystalline titanium silicates are associated with the charged titania chains and not the uncharged rings, which form the bulk of the structure. In ETS-10, this association of cations with the charged titania chains is widely recognized as resulting in the unusual thermodynamic interactions with a wide variety of sorbates, which have been found. This includes relative weak binding of polar species such as water and carbon dioxide and relatively stronger binding of larger species, such as propane and other hydrocarbons. These thermodynamic interactions form the heart of low temperature desiccation processes as well as evolving Claus gas purification schemes. The unusual sorbate interactions are derived from the titanosilicate structure, which places the counter-balancing cations away from direct contact with the sorbates in the main ETS-10 channels.
In recent years, scores of reports on the structure, adsorption and, more recently, catalytic properties of wide pore, thermally stable ETS-10 have been made on a worldwide basis. This worldwide interest has been generated by the fact that ETS-10 represents a large pore thermally stable molecular sieve constructed from what had previously been thought to be unusable atomic building blocks.
Although ETS-4 was the first molecular sieve discovered which contained the octahedrally coordinated framework atoms and as such was considered an extremely interesting curiosity of science, ETS-4 has been virtually ignored by the world research community because of its small pores and reported low thermal stability. As synthesized, ETS-4 has an approximately 4 xc3x85 effective pore diameter. Reference to pore size or xe2x80x9ceffective pore diameterxe2x80x9d defines the effective diameter of the largest gas molecules significantly adsorbed by the crystal. This may be significantly different from, but systematically related to, the crystallographic framework pore diameter. For ETS-4, the effective pore is defined by eight-membered rings formed from TiO62xe2x88x92 octahedra and SiO4 tetrahedra. This pore is analogous to the functional pore defined by the eight-membered tetrahedral metal oxide rings in traditional small-pored zeolite molecular sieves.
The pores of ETS-4 formed by the eight-membered polyhedral TiO6 and SiO4 units are non-faulted in a singular direction, the b-direction, of the ETS crystal and, thus, fully penetrate the crystal, rendering the ETS-4 useful for molecular separations. Recently, however, researchers of the present assignee have discovered a new phenomenon with respect to ETS-4. In appropriate cation forms, the pores of ETS-4 can be made to systematically shrink from slightly larger than 4 xc3x85 to less than 3 xc3x85 during calcinations, while maintaining substantial sample crystallinity. These pores may be xe2x80x9cfrozenxe2x80x9d at any intermediate size by ceasing thermal treatment at the appropriate point and returning to ambient temperature. These materials having controlled pore sizes are referred to as CTS-1 (contracted titanosilicate-1) and are described in commonly assigned U.S. Pat. No. 6,068,682, issued May 30, 2000 herein incorporated by reference in its entirety. Thus, ETS-4 may be systematically contracted under appropriate conditions to CTS-1 with a highly controllable pore size in the range of 3-4 xc3x85. With this extreme control, molecules in this range may be separated by size, even if the sizes of the respective molecules are nearly identical. This profound change in adsorptive behavior is accompanied by systematic structural changes as evidenced by X-ray diffraction patterns and infrared spectroscopy. The systematic contraction of ETS-4 to CTS-1 to a highly controllable pore size has been named the Molecular Gate(trademark) effect. This effect is leading to the development of separation of molecules differing in size by as little as 0.1 Angstrom, such as N2/O2 (3.6 and 3.5 Angstroms, respectively), CH4/N2 (3.8 and 3.6 Angstroms), or CO/H2 (3.6 and 2.9 Angstroms). High pressure N2/CH4 separation systems are now being developed.
Separations of fluid mixtures (gases or liquids) by adsorption utilizing the ETS-type molecular sieves have been proposed in which the molecular sieve is utilized in the form of a bed, typically fixed, through which the mixture to be separated flows. Both pressure swing adsorption (PSA) and thermal swing adsorption (TSA) have been suggested to effect separation of one or more fluids from mixtures containing same. Presently suggested separations using ETS molecular sieve adsorbents include the use of ETS-10 to adsorb hydrocarbon species from a Claus feed gas also containing hydrogen sulfide and other polar gases. In such process, the ETS-10 adsorbent is regenerated by a temperature swing (TSA) causing desorption of the hydrocarbons. Also proposed by the present assignee is the use of ETS-4 and contracted versions thereof, CTS-1, in a high pressure separation of nitrogen from natural gas. In this latter system, pressure swing adsorption (PSA) is utilized to adsorb the nitrogen from the natural gas stream, and desorb the nitrogen from the titanium silicate molecular sieve.
The unique property of ETS-10 to only weakly bind polar species so as to cause polar species to pass through the adsorbent at mildly elevated temperatures, and the ability to actually control and systematically shrink the pore size of ETS-4 to its CTS version have played significantly in allowing high capacity, fixed bed separation systems to be developed utilizing these titanium silicate molecular sieves. One disadvantage, however, of the PSA and TSA systems is that the adsorbent beds quickly reach the sorbent capacities thereof resulting in a xe2x80x9cbreakthroughxe2x80x9d of the sorbate into the product stream. An additional disadvantage of these processes is that at elevated temperatures, the adsorbent bed loses its capacity to hold the sorbate, resulting in the contamination of the product stream as the non-adsorbed sorbate passes through the spaces between the individual particles of the molecular sieve and breaks through into the product. Heating, however, is often advantageous to improve the kinetics of the adsorption process. Accordingly, adsorption undertaken in the presence of a bed of molecular sieve operates under multiple timed cycles of adsorption and desorption to prevent over-reaching the capacity of the adsorbent and consequent breakthrough of the sorbate into the product during adsorption, and contamination of the sorbate by product fronts during desorption. On a daily basis, many of such adsorption/desorption cycles must be run. Typically, multiple beds of molecular sieve are used, operating in parallel, some of which are undergoing adsorption while others undergo desorption or intermediate pressurizations and depressurizations if a PSA system is utilized. The need for multiple cycles and/or multiple beds obviously requires a high capital investment for production of a significant volume of product such as on a commercial scale. Moreover, such systems have high maintenance costs.
The use of membranes to provide fluid separation of mixtures is a known alternative to the use of beds of molecular sieves and use thereof in PSA or TSA processes. The membrane separation process is rather straight forward and does not require the timed cycles of adsorption and desorption needed with fixed bed molecular sieve technology. In membrane applications, small molecules (permeate) are not adsorbed but simply pass across the plane of the membrane through distinctly sized pores. The larger sized molecules (retentate) cannot pass through the pores and are retained upstream of the membrane plane. Accordingly, there is no adsorbent over-capacity problem and consequent breakthrough of retentate into product, and, thus, no need for timed cycles. There are, however, disadvantages to membrane technology. For one, while advances in polymer membranes have been made, these materials are still subject to chemical destabilization and are not universally inert to all fluid mixtures. Even water present can degrade many such membranes. Zeolite crystalline aluminosilicates with a narrow distribution of pore sizes on a molecular scale have high thermal, chemical, and mechanical stabilities. Molecular sieves can be, for example, alumina phosphates (ALPO) or silicoaluminophosphates (SAPO), which are also microporous, crystalline materials with a narrow distribution of pore sizes and also have high thermal, chemical, and mechanical stabilities. Therefore, zeolites and molecular sieves can be used in bed form not only for fluid separations in adsorption/desorption processes, as mentioned above, but also as diffusion membranes when prepared in thin film form. The size and adsorption properties of the zeolite pores, however, limit what can be separated with a particular type of zeolite membrane, even if the crystalline structure is perfect and defect free. Zeolite membranes are further problematic with respect to polar species, which are strongly held within the charged structure of the zeolite pores. Thus, fluid mixtures containing water, CO2, etc. can adversely affect membrane productivity. The simplification and, thus, lower capital and operational costs of separations utilizing membranes, however, wherein the permeate and retentate are continuously separated is a large factor in the continued development of membrane separations.
In accordance with this invention, separation of components from gaseous or liquid mixtures containing same is provided by contacting the mixtures with membranes formed from titanium silicate molecular sieves, including the ETS molecular sieves developed by Engelhard Corporation. The ETS sieves are distinguished from other molecular sieves by possessing octahedrally coordinated titania active sites in the crystalline structure. These molecular sieves contain electrostatically charged units that are radically different from charged units in conventionally tetrahedrally coordinated molecular sieves such as in the classic aluminosilicate zeolites. Members of the ETS family of sieves include, by way of example, ETS-4 (U.S. Pat. No. 4,938,939), ETS-10 (U.S. Pat. No. 4,853,202), and ETAS-10 (U.S. Pat. No. 5,244,650), all of which are titanium silicates or titanium aluminum silicates. The disclosures of each of the listed patents are incorporated herein by reference.
Membranes formed from ETS-4 molecular sieve are particularly useful inasmuch as the pores of the ETS-4 membranes can be systematically contracted under thermal dehydration to form CTS-1-type materials as disclosed in U.S. Pat. No. 6,068,682. Under thermal dehydration, the pore size of ETS-4 can be systematically controlled from about 4 xc3x85 to 2.5 xc3x85 and sizes therebetween and frozen at the particular pore size by ending the thermal treatment and returning the molecular sieve to ambient temperature. The ability to actually control the pore size of a particular molecular sieve greatly increases the number of separations achievable by a single molecular sieve unlike previous zeolite membranes in which the adsorption and diffusion properties of the zeolite pores limit what can be separated with a particular type of zeolite membrane. It has recently been discovered that certain polymorphs of ETS-4 can be prepared which not only contain the open small pores along the b-axis of the crystallographic lattice, which characterize ETS-4, but which further contain larger pores which are open and interpenetrate the lattice in the c-direction. Controlled shrinkage of these larger pores further increases the number of molecules which can be separated by this polymorph-enriched ETS-4. This material has been called ETS-6 and is the subject of co-pending application U.S. Ser. No. 09/640,313, filed Aug. 15, 2000.
The titanium silicate membranes of this invention are prepared by methods known in the art, such as by processes used for preparing aluminosilicate zeolite membranes.